1. Field of the Invention
The present invention relates to a plasma generating apparatus for deposition or etching of thin films in semiconductor circuit devices or the like.
2. Description of the Related Art
In processing of thin films in which lines of submicron widths are formed, such as in semiconductor circuit devices, a shift has taken place from wet etching in which a solution is used to dry etching in which plasma is used, in order to improve the processing accuracy. The plasma is required to have a high degree of purity in order to effect high-accuracy processing. If arc discharge or the like is used as a plasma generating apparatus, metallic impurities are mixed in the plasma, so that high-frequency electric fields of microwaves or the like have come to be used increasingly without using electrodes. For instance, in Japanese Patent Unexamined Publication No. 56-152969, microwaves are introduced into a plasma generating chamber through a waveguide to generate plasma, electromagnets are disposed around the plasma generating chamber to form in the plasma chamber magnetic fields having the same direction as the advancing direction of the microwaves, and electron cyclotron resonance is made to take place in the chamber so as to produce the plasma effectively. In addition, positive ions are taken out through ion extraction electrodes while they are being accelerated, and this ion beam is used to effect deposition, etching or the like of thin films. In this case, however, the intensity of magnetic fields produced by the electromagnets is substantially uniform in the radial direction inside the plasma chamber, while the intensity of the electromagnetic fields of microwaves led through the waveguide is strong in the axis of the plasma chamber and is weaker toward the side wall of the plasma chamber. Accordingly, plasma is generated mainly in a central portion of the plasma generating chamber and is diffused toward the periphery thereof, is collided against the side walls, and is hence recombined. Therefore, the distribution of the density of the plasma in the plasma generating chamber is thick in the central portion, and that of the ion beam taken therefrom through the extraction electrodes become non-uniform. This non-uniformity is undesirable in the deposition or etching of thin films. Furthermore, the cross-sectional area of the plasma chamber cannot be made very large since it is restricted by the cross-sectional area of the waveguide in the light of the propagation efficiency of the microwaves. Although a large diameter of the plasma generating chamber would be advantageous in that a large number of workpieces can be processed simultaneously, it is impossible to obtain this advantage. Japanese Patent Unexamined Publication No. 56-112477 discloses means for generating plasma having a large diameter. In this means, however, the plasma is generated and expanded inside an internal conductor of a coaxial waveguide, so that impurities from a wall surface of the internal conductor are mixed in, and the purity of the plasma therefore declines. Furthermore, Japanese Patent Unexamined Publication No. 61-288348 provides the following arrangement: The plasma which is obtained by the discharge of microwaves, such as those described above, is used as a plasma cathode, a main plasma chamber is provided such as to be disposed adjacent to the plasma cathode, a cathode is provided at an inlet port of the main chamber adjacent to the plasma chathode, an anode is provided on a side wall of the main plasma chamber, electrons are taken out from the plasma cathode, a DC discharge is performed in the main plasma chamber, and a some part of gas to the chamber is converted into plasma, thereby obtaining plasma of a large diameter. In this arrangement, however, positive ions inside the main plasma chamber collide against the cathode, and sputtering consequently occurs, thereby disadvantageously resulting in a decline in the purity of the plasma.